|Publication number||US6816403 B1|
|Application number||US 10/249,876|
|Publication date||Nov 9, 2004|
|Filing date||May 14, 2003|
|Priority date||May 14, 2003|
|Also published as||US20040228170|
|Publication number||10249876, 249876, US 6816403 B1, US 6816403B1, US-B1-6816403, US6816403 B1, US6816403B1|
|Inventors||Ciaran J. Brennan, John K. DeBrosse, Russell J. Houghton|
|Original Assignee||International Business Machines Corporation|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (12), Non-Patent Citations (3), Referenced by (53), Classifications (9), Legal Events (6)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present invention relates generally to magnetic memory devices and, more particularly, to a capacitively coupled sensing apparatus and method for cross point magnetic random access memory (MRAM) devices.
Magnetic (or magneto-resistive) random access memory (MRAM) is a promising technology in the development of non-volatile random access memory that could begin to replace the existing dynamic random access memory (DRAM) as the standard memory for computing devices. The use of MRAM as a non-volatile RAM will eventually allow for “instant on” systems that come to life as soon as the system is turned on, thus saving the amount of time needed for a conventional PC, for example, to transfer boot data from a hard disk drive to volatile DRAM during system power up.
A magnetic memory element (also referred to as a tunneling magneto-resistive, or TMR device) includes a structure having ferromagnetic layers separated by a non-magnetic layer, and arranged into a magnetic tunnel junction (MTJ). Digital information is stored and represented in the memory element as directions of magnetization vectors in the magnetic layers. More specifically, magnetic vectors in one magnetic layer (also referred to as a reference layer) are magnetically fixed or pinned, while the magnetization direction of the other magnetic layer (also referred to as a “free” layer) may be switched between the same direction and the opposite direction with respect the fixed magnetization direction of the reference layer. The magnetization directions of the free layer are also known “parallel” and “antiparallel” states, wherein a parallel state refers to the same magnetic alignment of the free and reference layers, while an antiparallel state refers to opposing magnetic alignments therebetween.
Depending upon the magnetic state of the free layer (parallel or antiparallel), the magnetic memory element exhibits two different resistances in response to a vertically applied current with respect to the TMR device. The particular resistance of the TMR device thus reflects the magnetization state of the free layer, wherein resistance is “low” when the magnetization is parallel, and “high” when the magnetization is antiparallel. Accordingly, a detection of changes in resistance allows an MRAM device to provide information stored in the magnetic memory element (i.e., a read operation). In addition, an MRAM cell is written to through the application of a bi-directional current in a particular direction, in order to magnetically align the free layer in a parallel or antiparallel state.
However, one difficulty with the practical operation of a cross-point MRAM array relates to the sensing of a particular cell, given that each cell in the array is coupled to the other cells through several parallel leakage paths. The resistance seen at one cross point equals the resistance of the memory cell at that cross point in parallel with resistances of memory cells in the other rows and columns. If the memory cell being sensed has a different resistance due to the stored magnetization, a small differential voltage may develop. This small differential voltage in turn can give rise to a parasitic current, which is typically much larger than the sense current, and thus can obscure the sensing of the sense current and hence the resistance of the cell. As a result, complex auto-zeroing techniques have been implemented in conventional sensing schemes to subtract out the error voltage or the error current. Moreover, because the selected bit line voltage is controlled by a feedback amplifier, the stable operation thereof requires a compensation capacitance, which results in a slower read access time.
The foregoing discussed drawbacks and deficiencies of the prior art are overcome or alleviated by a sensing apparatus for a cross point magnetic random access memory (MRAM) device. In an exemplary embodiment, a sense amplifier is selectively coupled to a selected bitline, the selected bitline being in communication with a selected MRAM cell to be read, and the selected MRAM cell further in communication with a selected wordline associated therewith. A reference current source is coupled to the selected bitline. The sense amplifier is configured to receive as an input thereto a signal voltage generated on said selected bitline, the signal voltage being generated in response to a reference current supplied by the reference current source and a read current applied through the selected MRAM cell. The sense amplifier is further configured to provide an offset corrected, amplified output reflective of the data state of the selected MRAM cell.
In another aspect, a method for sensing data stored within a cross point magnetic random access memory (MRAM) device includes establishing an offset voltage of a sense amplifier, the sense amplifier selectively coupled to a selected bitline within the MRAM device, the selected bitline being in communication with an MRAM cell to be read. A read current is applied through the MRAM cell to be read, and a reference current is applied through the selected bitline. A signal voltage is sensed on the selected bitline, the signal voltage being generated in response to the read current and the reference current. The signal voltage is coupled to an input of the sense amplifier, wherein the sense amplifier provides an offset corrected output reflective of the data state of the MRAM cell.
In still another aspect, a method for sensing data stored within a cross point magnetic random access memory (MRAM) device includes coupling a sense amplifier to a selected bitline, the selected bitline being associated with a plurality of MRAM cells defined at intersections of the selected bitline and a plurality of wordlines. A shorting device within said sense amplifier is activated and deactivated so as to store a DC offset value associated with the sense amplifier. The plurality of wordlines and at least one reference wordline are initially biased at an equalization voltage, and the voltage on a selected one of the plurality of wordlines is altered from the equalization voltage, thereby causing a read current to flow through a selected MRAM cell. The voltage on the at least one reference wordline is also altered from the equalization voltage, thereby causing a reference current to flow through the selected bitline. A signal voltage developed on the bitline is amplified, the signal voltage having a polarity reflective of the data state of selected MRAM cell, wherein the amplified signal voltage is offset corrected by the sense amplifier.
Referring to the exemplary drawings wherein like elements are numbered alike in the several Figures:
FIG. 1 is a schematic diagram of a conventional sensing scheme for a cross-point MRAM device;
FIG. 2 is a schematic diagram of a capacitively coupled sensing apparatus for a cross-point MRAM device, in accordance with an embodiment of the invention;
FIG. 3 is a schematic diagram of one possible embodiment of the reference current source shown in FIG. 2;
FIG. 4 is a schematic diagram of one possible embodiment of the sense amplifier shown in FIG. 2; and
FIG. 5 is a timing diagram illustrating an embodiment of a method for sensing data in a cross point MRAM device, in accordance with the sensing scheme illustrated in FIGS. 2 through 4.
Referring initially to FIG. 1, there is shown a schematic diagram of a conventional sensing scheme 100 for a cross-point MRAM device. A differential sense amplifier 102 is selectively coupled to a selected bitline (BL) 104 through a column select device 106 (e.g., a field effect transistor (FET)), for the purpose of sensing or reading data stored in an MRAM cell 107 at the junction of the selected bitline 104 and a selected wordline (WL) 108. The column select device 106, when activated, couples the selected bitline 104 to a sense line (SL) 110, which in turn is coupled to the differential sense amplifier 102. In addition, the sense line 110 may be fanned out to a number of bitlines, such that a multiplexing scheme is used to connect any of the bitlines to the sense line 110.
In operation, the sensing scheme 100 of FIG. 1 determines a resulting bitline current through the MRAM cell when a voltage differential is applied between the selected bitline 104 and the selected wordline 108. As is known in the art, the resistance of the MTJ of an MRAM cell (represented by Rsel in FIG. 1) varies in accordance with the value of the data stored therein. Thus, by measuring the value of the resulting bitline current in response to a voltage thereacross, the value of the cell data is determined. More specifically, the unselected wordlines 112 are held at an equalization voltage (Veq), while the voltage of the selected wordline 108 is lowered by an applied voltage (Va) to a value of about Veq−Va. The differential amplifier 102, being configured in a negative feedback fashion, attempts to hold the voltage on the selected bitline 104 at Veq. Ideally, the only current then flowing through the selected bitline 104 would be the current Isel passing through the selected cell (given by Va/Rsel), since both the unselected wordlines 112 and the selected bitline 104 would be at Veq. Thus, the unselected cells along the bitline 104 would have no differential voltage applied thereacross.
In reality, however, there is always at least some small offset error in the voltage applied to the selected bitline 104 (Veq+Verr). This offset error voltage, for the most part, is due to device threshold mismatch within the differential amplifier 102 controlling the selected bitline voltage. As a result, the error voltage will create an error current Ierr passing through the unselected cells along the selected bitline 104. Unfortunately, in order for this error current to be small relative to the signal current (so as to be able to conveniently detect the correct selected cell current), the error voltage must be well below the device threshold mismatch tolerances associated with a CMOS process (which are typically on the order of about 5 to 10 millivolts).
Therefore, in accordance with an embodiment of the invention, there is disclosed a capacitively coupled sensing apparatus and method for cross point magnetic MRAM devices. Briefly stated, a reference current is applied to the selected bitline and a read current is applied to the data cell to be read, thereby generating a signal voltage on the selected bitline in response to the reference current, the current representative of the data state and the offset current. The selected bitline is further coupled to a sense line in signal communication with a capacitively coupled inverting amplifier that effectively compensates for any offset resulting from individual device threshold mismatching.
Referring now to FIG. 2, there is shown a schematic diagram of a novel sensing apparatus 200, in which a sense amplifier 202 is selectively coupled to a selected bitline 204 through column select device 206 and sense line 207. As is described in greater detail hereinafter, the sense amplifier 202 is a capacitively coupled, inverting voltage amplifier 208 that utilizes a shorting device 210 (e.g., an FET) to establish the DC operating point thereof. The DC offset is stored on capacitor 212. In addition, a reference current source 214 is used to provide a reference current onto the selected bitline 204. By designing the reference current source to provide a reference current on the selected bitline 204 at a value between the two current levels of opposite data states, the resulting net current through the selected bitline 204 will have a polarity reflective of either a “0” data bit or a “1” data bit stored within the selected cell.
In order to generate the reference current (Iref) a pair of reference wordlines are configured, wherein a first of the reference wordlines 302 includes reference cells that are all preconditioned to a “0” data state, while a second of the reference wordlines 304 includes reference cells that are all preconditioned to a “1” data state, as illustrated in FIG. 3. In other words, the reference cells in the first reference wordline 302 will have a collective first resistance R0, while the reference cells in the second reference wordline 304 will have a collective second resistance R1 such that the average of two currents flowing therethrough is sent through the selected bitline 204.
FIG. 4 is a schematic diagram of one possible embodiment of the sense amplifier 202 shown in FIG. 2. A CMOS inverter 402 is used as an inverting amplifier (i.e., an NFET and PFET in a common source configuration), which is capacitively coupled to the sense line 207 through capacitor 212, as stated previously. In addition, another CMOS inverter 404 is used to provide an amplified inverted reference voltage signal, being capacitively coupled to Veq. The source voltages of the NFETs of both inverters 402,404 are tied to Veq in order to improve immunity, while the source voltage of the PFETs (Vpp) is selected to maximize the voltage gain of the amplifier. As is the case with the shorting device 210, the reference inverting amplifier 404 also includes a shorting device 406 across the input and output thereof.
The output of the inverting voltage amplifier 402 drives one side (Out) of a CMOS cross-coupled latch 408, while the other side thereof (bOut) is driven by the reference inverting amplifier. Initially, the supply nodes (GND, VDD) of the latch 408 are allowed to float. Then, after a sufficient signal is allowed to develop across the latch 408, access NFET (Set) and PFET (bSet) are activated to couple the latch supply nodes to the power source thereof (GND and VDD, respectively), to further amplify the sense line signal to a full CMOS level. It will be appreciated that since the signal at the output of the inverting amplifiers 402, 404 has been amplified, any effects due to device threshold mismatch between the inverting voltage amplifiers 402, 404 (or between the cross-coupled inverters of the latch 408) will be substantially eliminated.
FIG. 5 is a timing diagram illustrating an embodiment of a method for sensing data in a cross point MRAM device, in accordance with the sensing scheme illustrated in FIGS. 2 through 4. A read cycle is commenced with the coupling of the selected bitline 204 to the sense line 207, when the column select device 206 is energized (the unselected bitlines may be allowed to float). This is reflected at time t1 in FIG. 5, wherein the “column select” signal goes from low to high. Prior thereto, the “shorting device” signal remains at a high level, signifying the shorting of the sense amplifier 202 output and input. In addition, the selected wordline and reference wordlines are initially biased at the equalization voltage, Veq. The unselected wordlines and the selected bitlines settle to a voltage equal to Veq. At time t2, and after the amplifier DC offset has been stored on capacitor 212, the shorting device 210 is deenergized. Since the coupling capacitor 212 of the sense amplifier 202 has stored the DC offset, the next phase of the reading operation may be implemented.
At time t3, the selected WL is driven to Veq minus the applied voltage (Va) or Veq−Va. Concurrently, the reference current is applied to the bitline by driving the voltage on each of the reference lines from Veq to Veq+Va/2. As a result of both the applied reference current and the data-dependent current through the selected MRAM cell (Isel), a net current is produced through the selected bitline, the polarity of which depends on the state of the data in the cell, since this determines the value of Rsel. Depending upon the polarity of the net current (denoted by Iref−Isel in FIG. 3), the selected bitline and sense line settle to a voltage slightly above or below Veq by an amount designated by Vsig. As shown in FIG. 5, the value of the voltage on the selected bitline increases to Veq+Vsig in one data state or (as indicated by the dashed line) decreases to Veq Vsig. The specific value of Vsig is accurately approximated by (Iref−Isel)*Runsel, where Runsel is the combined parallel resistance of the unselected cells along the selected bitline.
In response to the change in voltage of the selected bitline, the corresponding output of the inverting voltage amplifier responds by falling or rising from the DC offset voltage (Vdc) by a voltage equal to Vsig multiplied the voltage gain (A) of the inverting voltage amplifier. Accordingly, the solid line in the “out” signal in FIG. 5 decreases by an amount given by the expression Vdc+A*Vsig (with the gain “A” being a negative number for an inverting amplifier). Conversely, the dashed line in the “out” signal indicates an increase if the voltage on the selected bitline is increased. Finally, at time t4, the “set” signal in FIG. 5 is increased from low to high, thereby coupling the supply nodes of CMOS latch 408 to their respective rail voltages, which results in the signal on the sense amplifier output to be increased to either VDD or decreased to GND.
As will be appreciated, both the above described sensing scheme and method may be implemented through several other possible embodiments. For example, the column select device 206 could be replaced by a direct coupling of each bitline to a separate sense line and sense amplifier. Although this would entail additional device area, the need for multiplexing a column select signal for several bitlines sharing a common sense amplifier is eliminated. In another possible embodiment of the sense amplifier 202, the outputs of the inverting voltage amplifiers 402, 404 could drive the inputs of a differential amplifier, as opposed to the CMOS cross-coupled latch 408 of FIG. 4.
Alternative embodiments are also contemplated with regard to the reference current source 214. For example, in lieu of a pair of reference wordlines (302, 304) having reference cells set to opposite data states, a single reference wordline 306 and having a collective resistance R2 (shown in phantom in FIG. 3), with each cell thereon conditioned to a known state could also be used. In order to generate the same magnitude of reference current (Iref) as shown in FIG. 3, the single reference wordline 306 is driven to a voltage Veq+Va′, wherein Va′ is selected such that the current through the reference cell 306 is equal to the average of the currents flowing through two selected cells of opposite data states. Furthermore, the location of the reference current source need not be dependent on the location of the selected bitline. In other words, the reference current could be directly forced into or out of the sense line instead of the selected bitline. Moreover, the reference current source 214 need not be created through a reference wordline(s)/cell(s); a conventional current source may also be implemented.
In the exemplary embodiment of FIGS. 2-4, the sense line 207 settles to Veq before the shorting device 210 is turned off. Equivalently, the sense line 207 could be held at Veq by a separate precharge device, with the activation of the column select device 210 being delayed until after the shorting device 210 is turned off.
This effectively multiplexes the sense amplifier input from Veq to the selected bitline 204. Both the selected wordline and the reference current could be activated throughout the entire read cycle in such a case. Alternatively, a multiplexer circuit could be disposed between the sense lines and the sense amplifier in order to switch the sense amplifier input from Veq to Veq+Vsig after the shorting device is deactivated. This would also allow the selected wordline and the reference current to be activated throughout the entire read cycle.
As is the case with the particular sensing apparatus embodiments outlined above, variations within the sensing read/cycle are also contemplated. For example, rather than allowing the unselected bitlines to float, they could also be driven to Veq. Also, the polarity of the applied voltages to the selected and reference wordlines could be reversed such that the selected wordline voltage is driven to Veq+Va, while the reference wordline are driven to Veq−Va/2. Stated another way, the applied voltage Va may be either positive or negative, so long as this value is added/subtracted with respect to the selected wordline and subtracted/added with respect to the reference wordlines.
In the exemplary embodiment described in FIG. 5, both the reference current (Iref) and the selected wordline current (Isel) are sensed simultaneously by applying the corresponding voltages to the reference wordlines and selected wordline once the shorting device is deactivated. However, this sequence could be reversed by activating the selected wordline and reference wordline voltages prior to deactivating the shorting device, and then restoring the applied voltages to the selected wordline and reference wordlines to Veq after the shorting device is turned off. Alternatively, a sequential sensing technique could be implemented wherein either Iref or Isel is sensed prior to deactivating the shorting device of the sense amplifier.
Regardless of the particular embodiment(s) implemented, present MRAM sensing scheme and method provides several advantages over a conventional sensing scheme, in that the sensed signal Vsig is independent of the base value of the MTJ resistance, which in turn is strongly dependent upon several process variables. Moreover, this sensing is accomplished while also substantially eliminating sensitivity to device threshold. Furthermore, since the amplifier is a single pole amplifier, there are essentially no stability concerns associated therewith, resulting in a potentially faster read access time.
While the invention has been described with reference to a preferred embodiment or embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
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|U.S. Classification||365/158, 365/205, 365/207|
|International Classification||G11C7/00, G11C7/02, G11C11/16, G11C11/00|
|May 14, 2003||AS||Assignment|
Owner name: INTERNATIONAL BUSINESS MACHINES CORPORATION, NEW Y
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:BRENNAN, CIARAN J.;DEBROSSE, JOHN K.;HOUGHTON, RUSSELL J.;REEL/FRAME:013652/0795;SIGNING DATES FROM 20030414 TO 20030505
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